In the process of producing alkali metal hydroxide and chlorine by electrolyzing an alkali metal chloride brine, such as a sodium chloride brine or a potassium chloride brine, in a diaphragm cell, the alkali metal chloride is fed to the anolyte chamber, a voltage is imposed across the cell, and chlorine is recovered from the anolyte chamber while the alkali metal hydroxide, present in a cell liquor with the alkali metal chloride, and hydrogen are recovered from the catholyte chamber. The overall anode reaction is: EQU 2Cl.sup.- .fwdarw. Cl.sub.2 + 2e.sup.- (1)
While the overall cathode reaction is: EQU 2H.sub.2 O + 2e.sup.- .fwdarw. H.sub.2 + 2OH.sup.-. (2)
more precisely, the cathode reaction is reported to be EQU H.sub.2 O + e.sup.- .revreaction. H.sub.ads + OH.sup.- (3)
by which the monatomic hydrogen is adsorbed onto the surface of the cathode. In basic media, the adsorbed hydrogen is reported to be desorbed according to one of two processes: EQU 2H.sub.ads .revreaction. H.sub.2 ( 4) EQU h.sub.ads + H.sub.2 O + e.sup.- .revreaction. H.sub.2 + OH.sup.-. (5)
the hydrogen desorption step, i.e., reaction (4) or reaction (5), is believed to be the hydrogen overvoltage determining step. That is, it is the rate controlling step and its activation energy corresponds to the cathodic hydrogen overvoltage. Typically, the electrode potential for the overall reaction (2) is on the order of about 1.1 volts on steel in basic media. Steel, as used herein to characterize cathodes, includes iron and iron alloys useful as chlor-alkali cell cathodes.
However, it has now been found that the hydrogen overvoltage may be reduced, for example, by from about 0.05 volt to about 0.20 volt by adding an hydroxy carboxylic acid, preferably in combination with an organic phosphonate and a surfactant, to the catholyte liquor of the cell.